394 COMPARATIVE ANATOMY 



under the control of the nervous system : the capillaries again 

 unite to form the factors of the veins. The walls both of veins 

 and arteries consist, in addition to the epithelium, of connective 

 and elastic tissue and of unstriated muscular fibres, and are much 

 thicker in the case of the arteries than in that of the veins, in 

 some of which the muscular elements may be altogether wanting. 



Both blood and lymph consist of a colourless fluid, the plasma, 

 in which float numerous cells or corpuscles. The blood-corpuscles are 

 of two kinds colourless, nucleated, amoeboid cells, known as white 

 or colourless corpuscles or leucocytes, and far more numerous red 

 Hood-corpuscles or erythrocytes. 1 The colour of these is due to 

 haemoglobin, which readily enters into loose chemical combination 

 with oxygen, and they are the specific respiratory cells. They have 

 no longer, however, the characteristic structure of protoplasm, and 

 are always surrounded by a membrane. The lymph contains 

 leucocytes only ; these are similar to those of the blood, and are 

 sometimes also spoken of as phagocytes? 



The nuclei of the red corpuscles persist, and the whole cell is 

 biconvex, in all Vertebrates below Mammals ; and, even in these 

 nucleated red cells may be seen in the marrow of the bones (in 

 which more especially they are formed throughout life), in the 

 blood of the spleen, and often in that of the portal vein : in all 

 other parts of the body of Mammals they lose their nuclei. 

 In all Mammals, except the Camelidee, the red corpuscles are seen to 

 have the form of circular, biconcave discs; 3 in the last-mentioned 

 family and in all other Vertebrates except Cyclostomes they are 

 oval. They are largest in certain Urodeles, being in Amphiuma as 

 much as 75/i in their longest diameter; then come, in order, other 

 Urodeles and Dipnoans, Reptiles, Anurans, Fishes, Birds, and 

 Mammals, in the last-mentioned of which they are the smallest, 

 varying in different families from 2'5/A (Tragulidse) to lOyu,. 



The heart is enclosed within a serous membrane, the pericar- 

 dium (Fig. 295), which, as already mentioned, consists of parietal 

 and visceral layers. In most Anamnia and in early embryos of 

 higher forms it is situated close to the head, but on the differ- 

 entiation of a neck, comes to lie relatively further back. It arises 

 either as a single (Cyclostomi, Elasmobranchii, Ganoidei, 

 Amphibia) or as a paired (Teleostei, Sauropsida, Mammalia) 

 tubular cavity in the splanchnic layer of the mesoderm along the 

 ventral region of the throat, close behind the gill-clefts, and the 

 part of the coelome around it gives rise to the pericardial cavity. 

 Its wall becomes differentiated into three layers, an outer 



1 In Amphioxus the blood contains no formed elements. 



2 In addition to the leucocytes and erythrocytes, a third kind of corpuscle 

 occurs in the blood : these structures are known as Uood-plates or thromhocytes. 

 Each has the form of a minute, flat disc, is colourless and ama-boid, and consists 

 of nucleated protoplasm. It is very possible that they are derivatives of the red 

 and white corpuscles. In coagulation of the blood they undergo characteristic 

 changes. 



3 They are said to be primarily cup-shaped. 



